1. Field of the Invention
The present invention relates, in general, to a polyester prepolymer and, more particularly, to thermosetting polyester prepolymer showing shape-memory effect.
2. Description of the Prior Art
As well known to those skilled in the art, both some alloys and some synthetic resins show shape-memory effect. Known shape-memory alloys are, for example, titanium-nickel alloy, copper-zinc-aluminum alloy and the like. However, the shape-memory alloys are such metals that they are not easy to process and not suitable for mass production of resulting products. Another problem of the shape-memory alloys is in that the shape-memory alloys not only have high specific weight but also are so expensive that they are scarcely generally used. Meanwhile, the shape-memory resin is easily formed, light in weight and shows good processing performance, so that it is known to be suitable for used in mass production of resulting products even though the shape-memory effect of the known shape-memory resin is somewhat less than that of the known shape-memory alloys. The shape-memory resin, after being deformed, recovers to its original shape when it is heated. That is, when the shape-memory resin, after being deformed, is heated to temperatures higher than a predetermined level, the stress imposed on the shape-memory resin when the shape-memory resin was deformed is relieved, thus making the shape-memory resin recovering its original shape.
Japanese Patent Laid-open Publication No. Sho. 61-7336 discloses a method for the preparation of a known shape-memory resin. In accordance with a method of the above Japanese patent, a copolymer of polyamide and polyetheramide containing either multifunctional methacylate or acrylic monomer is crosslinked through radiation, thus to prepare a thermal shape-memory resin. However, the method disclosed in the above Japanese patent has a problem in that the method necessarily includes a post cross-linking step. The crosslinked polymer should become rubber phase so as to be provided with desired processability, however, which rubber phase polymer shows less rigidity around room temperature and are inevitably limited in its use. Another problem of the method of the above Japanese patent is that the resulting thermal shape-memory resin is difficult to mold, thus can be scarcely reused. In addition, it may include a permanently deformed part after heating for recovering the original shape.
Japanese Patent Laid-open Publication No. Sho. 59-535520 discloses polynorbornene that has both a glass transition point of not less than 10.degree. C. and a number average molecular weight of not less than one million. However, the polynorbornene disclosed in this Japanese patent still is a rubber phase material that shows less strength at about room temperature. Another problem of the above polynorbornene is resided in that the high molecular weight of the polynorbornene deteriorates the processability.
Japanese Patent Laid-Open Publication No. Sho. 62-275114 discloses a styrene-butadiene copolymer showing shape-memory effect. However, the styrene-butadiene copolymer has a problem in that the shape-memory effect, as the copolymer is repetitively subjected to deformation and shape recovering, is gradually deteriorated with time.
Japanese Patent Laid-Open Publication No. Sho. 62-192440 discloses a crosslinked trans-polyisoprene showing shape-memory effect. However, the crosslinked trans-polyisoprene, while showing good shape-memory effect, nevertheless is deteriorated in both weatherability and thermal aging resistance due to the diene bond, thus to be inevitably faced with deterioration of physical performance during use of the shape-memory polymer.